Several key challenges have limited zinc-iodine secondary energy storage devices from achieving the cost and performance levels predicted or expected for such devices. These can include, but are not limited to, an absence of low-cost, suitable cathode materials with high capacity and long-term cycle stability; shuttle effects and self-discharge problems associated with active iodine species being undesirably dissolved in the aqueous electrolyte and freed from the cathode; the necessity of ion exchange membranes to limit the problem of soluble iodine species; and instability and gas evolution problems. Accordingly, there exists a need for zinc-iodine secondary energy storage devices, methods, and electrolytes that can address the challenges.